Global Monsoon Working Group

Launch				Synthesis

2007	2009	2011	2013	2015

Fig. 1: Global Monsoon domains. Color shading shows the annual range of precipitation (summer minus winter normalized to annual mean). Black lines outline the Global monsoon precipitation domain, defined by an annual range greater than 2 mm/day and 70% of the annual mean precipitation. Red (Blue) line indicates the ITCZ position in Jan-Feb (July-August), estimated from the maximum precipitation rate (figure from Wang and Ding 2008).

Summary

The Global Monsoon Working Group was active from 2007 to 2015.

Goals

This working group aimed to incorporate a holistic approach to monsoon research within the paleo community.

Background information

The term monsoon generally refers to the seasonal reversal in atmospheric low-level circulation, particularly the surface winds and associated precipitation (i.e. wet summers and dry winters). These changes, occurring in the tropical and subtropical regions, arise from the seasonal migration of the Intertropical Convergence Zone and a reversal in northern and southern hemispheric heating and temperature gradients between the continent and the ocean, resulting in the alternation of wet and dry seasons.

The "global monsoon" concept refers to a global-scale persistent overturning of the atmosphere throughout the tropics and subtropics that varies with the time of year (Trenberth et al. 2000).

In the modern world, six monsoon systems are recognized: the African, South Asian, East Asian, Australian, North American, and South American monsoons (Fig. 1; Wang and Ding 2006; 2008), although the latter two have not been clearly identified with wind reversals (Webster et al. 1998).

Fig. 2: Global land monsoon precipitation AD 1948-2003 from 4 different global rainfall datasets. Downward trend due to weakening of the summer monsoon in Northern Hemisphere. (a) Northern Hemisphere averaged June-July-August precipitation, (b) the Southern Hemisphere-averaged December-January-February precipitation, and (c) the global monsoon index (GMI), or the sum of Figures 1a and 1b (Wang and Ding, 2006).

Because these regional monsoons are brought about by the same annual solar heating cycle, but differ from each other due to their geographic position and orographic features, changes in the global monsoon as a whole are now studied in addition to the regional monsoon systems. For example, the precipitation-based “Global Monsoon Index” was found to be declining from 1948 to 2003 over the land monsoon regions, primarily due to a weakening of the summer monsoon rainfall in the Northern Hemisphere, but increasing from 1979-2003 over the oceanic monsoon regions (including marginal seas) (Fig. 2. Wang & Ding 2006).